Solar cells are devices that convert the sun's energy into electricity using the photovoltaic effect. Solar power is an attractive energy source because it is sustainable and non-polluting. Accordingly, a great deal of research is currently being devoted to developing solar cells with enhanced efficiency while maintaining low material and manufacturing costs. Very simply, when photons in sunlight hit a solar panel, they are absorbed by semiconducting materials, such as silicon. Electrons are knocked loose from their atoms, allowing them to flow through electroconductive parts of the solar panel and produce electricity.
The most common solar cells are those based on silicon, more particularly, a p-n junction made from silicon by applying an n-type diffusion layer onto a p-type silicon substrate, coupled with two electrical contact layers or electrodes. In order to minimize reflection of the sunlight by the solar cell, an antireflection coating, such as silicon nitride, is applied to the n-type diffusion layer to increase the amount of light coupled into the solar cell. Using a silver paste, for example, a grid-like metal contact may be screen printed onto the antireflection layer to serve as a front electrode. This electrical contact layer on the face or front of the cell, where light enters, is typically present in a grid pattern made of “finger lines” and “bus bars” rather than a complete layer because the metal grid materials are not transparent to light. Finally, a rear contact is applied to the substrate, such as by applying a back side silver or silver/aluminum paste to the tabbing areas of backside followed by applying an aluminum paste to the remaining areas of the back surface. The device is then fired at a high temperature to convert the metal pastes to metal electrodes. A description of a typical solar cell and the fabrication method thereof may be found, for example, in European Patent Application Publication No. 1 713 093.
The conventional method for building the back side of a solar cell involves printing a dry back contact silver busbar, then printing dry aluminum to cover the rest of the back surface. A full aluminum back surface field (BSF) is desirable to improve solar cell performance. However, printing and drying aluminum to cover the entire back surface of a substrate and then printing and drying a silver back contact on top of the dried aluminum is not feasible because peeling of the silver film and the aluminum base support underneath are observed upon co-firing. Further, methods that involve printing, drying, and firing aluminum and then printing, drying, and firing silver busbars are also not feasible due to the lack of adhesion between silver and aluminum. Accordingly, a method in which silver and dried aluminum are co-fired would be desirable.
Aluminum BSF is the most economical process in the mass production of silicon based solar cells to form a back surface field and creating a low high junction, while acting as an impurity getter and providing partial surface passivation. However, the relatively poor solderability of aluminum is a barrier to forming a full BSF. Additionally, aluminum is loosely bonded by glass systems and thus does not create a firm base for printing metal connecting terminals. An improved method for applying aluminum BSFs would be desirable.